Machine tools are designed to produce movement of an object to be machined or inspected and/or a cutting tool (or grinding wheel) along at least one axis of motion, and typically three or more axes. For example, a “3-axis milling machine” typically moves any object to be machined along two orthogonal horizontal axes (“X” and “Y”) and moves a cutting tool spindle along a vertical third axis (“Z”) that is orthogonal to X and Y. A 4 or 5-axis milling machine adds one or two (respectively) rotary axes. An “A” axis provides a tilt angle around the X axis, a “B” axis provides a tilt angle around the Y axis, and a “C” axis provides a rotation around the Z axis. Each A, B, and C axis is orthogonal to the other two tilt axes. While three tilt axes could be added to a three axis milling machine to make a 6-axis milling machine, many machining jobs only require 5 axes of motion, so that is a common configuration. Additional motion axes may be added by providing axes that are parallel to each other but offset by a linear displacement. With the addition of such further axes, typical machine configurations are characterized as 7-axis machines and 9-axis machines. In addition, dimensional inspection machines closely resemble metal-removal machines except that the cutting tool (or grinding wheel) is replaced with a measurement probe.
Almost all turning machines (e.g., lathes and boring machines) provide at least two axes of relative motion between an object to be turned and a cutting tool. The “X” axis provides movement of the tool carriage perpendicular to the spindle (horizontally across the bed). An orthogonal “Y” axis provides vertical movement of a tool toward and away from the bed. An orthogonal “Z” axis provides movement of the carriage toward or away from the spindle chuck. Each of the X, Y and Z axes of a turning machine are orthogonal to the other two axes. Additional tool path and/or object motion axes may be provided by tilt axes A, B, and C. Lathe and boring machine tilt axes conform to the same standard as that for milling machines: The A axis provides a rotation around the X axis, the B axis provides rotation around the Y axis, and the C axis provides rotation around the Z axis.
Precision machining operations require accurate positioning of a cutting tool with respect to an object being machined. Existing machine tool performance analysis techniques are typically based upon position measurements along the X, Y, Z, A, B, or C axes that are taken under static or slowly moving operational conditions. While such tests may provide a useful assessment of some aspects of a machine tool's geometry errors, machine tools typically operate in modes where either the object being machined and/or the cutting tool move in rapid dynamic patterns. The term “dynamic” as used herein refers to conditions associated with relative acceleration and deceleration between two or more objects. The rapid dynamic patterns of motion are generated by a computer program, referred to herein as a “part program,” irrespective as to whether it is the object being machined that is being moved or the cutting tool that is being moved. The part program generates dynamic excitation commands that are provided to a machine tool's motion control system. The term “dynamic excitation commands” refers to motion commands that have an acceleration and/or a deceleration component. The motion controller is a programmable device and may be a microprocessor, a programmable logic controller, or a computer. The motion control system generates command signals that are amplified to drive motors that move various components of the machine tool.
Most existing machine tool performance analysis techniques do not detect the errors associated with a machine's dynamic characteristics, which are especially important in applications that approach the velocity and acceleration limits of a machine's servo system capabilities or when errors may be introduced by limits in the dynamic stiffness of a machine tool's structural frame. What are needed therefore are better methods to assess the accuracy and performance characteristics of machine tools under dynamic operational conditions.